1. Field of the Invention
The present invention relates to dual UV-curable and moisture curable silicone conformal coating compositions and methods of making and using same. In particular, the invention pertains to silicone conformal coating compositions having improved surface tack.
2. Description of the Related Art
Silicone materials are widely used in industry as well as in consumer markets as sealants, adhesives, conformal coatings, potting compounds, etc.
In particular, silicone conformal coatings have found widely accepted applications in the protection of delicate electronic components such as printed circuit boards as well as in abrasion resistant coatings for plastic lenses.
Conventionally, silicone conformal coatings are heat cured using hydrosilylation reactions, or are moisture cured using conventional room temperature vulcanizing.
In the heat cured coatings, silicone fluids with vinyl functional groups are reacted with silicone fluids containing silicon-hydrogen bonds in the presence of a transition metal catalyst, as for example chloroplatinic acid. These severe curing conditions along with the high cost of the catalysts present a serious limitation, however, on the usage of heat cured coatings.
Typically, moisture curable materials are manufactured by endcapping .alpha., .omega.-silanol terminated silicones with various crosslinkers such as alkoxysilanes, oximinosilanes, acetoxysilanes, aminosilanes, and other silanes with hydrolyzable groups attached to the silicon atom(s) thereof.
The resulting silicone materials are stored in moisture impermeable containers. During application to a respective substrate, the materials are extruded or otherwise applied and exposed to ambient conditions for curing. The moisture in the air then will hydrolyze the hydrolyzable groups (alkoxy, oximino, acetoxy, amino, etc.) on the silicon atom(s), to form silanol, either with or without the assistance of an added catalyst.
The resulting silanol can then further react with remaining unhydrolyzed groups in a condensation reaction, to form a siloxane linkage resulting in the cure of the silicone material.
Although these materials are very reliable and possess superior properties as coatings, the moisture cure tends to be slow. Overnight curing is often needed before a full cure can be achieved. This slow cure puts a severe limitation on the manufacture of coated components, since full cure of the coated components is needed before the components can be used in the next step of the manufacture process.
Because of the aforementioned limitations involved with heat or moisture curable silicone formulations, a third curing mode, ultraviolet light (uv) curing, has gained wide acceptance in recent years. The curing is relatively fast and mild to the substrates. In situations where portions of the coated material are shaded during the uv cure, a secondary cure mode, usually moisture cure can be further incorporated.
Typically, uv cure can be achieved by either a thiol-ene cure or by an acrylate cure. In the thiol-ene cure, a thiol functional silicone is reacted with a vinyl functional silicone. The cure is fast and the surface dry to the touch upon the completion of the cure. However, the finished coating resists heat aging poorly and the formulation tends to be storage unstable.
On the other hand, acrylate functional silicone is usually storage stable and the cured coatings exhibit excellent high temperature resistance. However, the acrylate cure typically exhibits oxygen inhibition. That is, in the presence of atmospheric oxygen, the surface cure tends to be incomplete and the resulting cured coating tends to be tacky.
EP 0539 234 pertains to a composition for a liquid gasket having both an ultraviolet-curing property and a moisture-curing property Which is prepared by combining a polysiloxane having at a molecular end at least one group which can be (meth)acryl, a polyorganosiloxane having silanol groups at both molecular ends, an organosilane having at least one hydrolyzable group at a molecular end and a photopolymerization catalyst and a condensation catalyst. The compositions disclosed in this reference are highly viscous, afford a bulk cure and are thus unsuitable for conformal coatings.
Japanese Patent Application No. 4-69901 by Three-Bond Co., Ltd. discloses an ultraviolet-curing, addition-polymerization-type silicone composition which is curable at room temperature. The disclosed silicone composition comprises a polydiorganosiloxane containing vinyl groups and a polydiorganosilane in which at least 4 silicon atoms are directly bonded in the formula R.sub.5 (Si(R.sub.3)(R.sub.4)).sub.n R.sub.6 in which R.sub.3 -R.sub.6 are alkyl or aryl, and n is 4 or greater, with the proviso that R.sub.5 and R.sub.6 may be directly bonded to one another. An addition polymerization catalyst is also included in the composition, such as a platinum catalyst. The composition is uv-light curable with the occurrence of radical addition polymerization.
Japanese Patent Application No. 92143102 to Tokyo Three Bond Co. Ltd. describes moisture curable and uv-curable compositions of a composition prepared by adding reactive polysiloxane to less equivalent reactive silane compounds having two or three hydrolysable groups, methacryl groups or epoxy groups and reaction of the composition with compounds having at least one vinyl group and methacryl groups and a moisture curing catalyst.
U.S. Patent No. 5,179,134 to Chu et al. describes an acryloxy-functional capped silicone which is formed as a reaction product of a silyl diacrylate compound and a silicone having at least one functionality which is reactive with an acryloxy functionality of the silyl diacrylate compound to yield the acryloxy-functional capped silicone.
Hoffman, V., et al., J. Mol. Struct., 293, 253-256 (1993) describes oligomers including a vinyl group-containing dimethylsiloxane which are thermally as well as photochemically crosslinkable.
Roth, W., et al., Adv. Mater., 2(10), 497-498(1990) discloses a methylvinyl-dimethylsiloxane composition that is devoid of photoinitiators, and undergoes radical crosslinking when exposed to laser radiation.
Barrall, E., et al., J. Polym. Sci., Polym. Symp., 71, 189-202 (1984) describes uv-cured polydimethylsiloxanes containing pendant vinyl groups, wherein the curing involves dicumyl peroxide-mediated vulcanization.
U.S. Pat. No. 4,064,027 to Gant describes a uv-curable composition consisting essentially of a vinyl-containing siloxane and siloxane-containing, silicon-bonded hydrogen atoms, in which a mercaptofunctional silicone may be employed as a cure accelerator.
U.S. Pat. No. 4,742,092 to Inoue et al. and U.S. Pat. No. 4,595,471 to Preiner et al. show UV and dual UV/moisture curable silicones which employ photocurable (meth)acryloxy and isopropenyloxy group containing silicones in combination with mercapto-containing silicones.
EP 0492828 discloses uv or moisture curable silicones incorporating additives of alkoxy terminated polydiorganosiloxanes in combination with an alkoxy silicone compound having photo reactive moieties.
Accordingly, it would be a substantial advance in the art, and is correspondingly an object of the present invention, to provide a dual uv-curable, moisture curable silicone conformal coating composition wherein uv cure of the composition results in a coating that is dry to the touch without the common tacky surface often associated with an acrylate cure.
Another object of the present invention is to provide an improved room temperature vulcanizing silicone conformal coating composition of such character, which is readily formulated in a convenient manner using conventionally available formulation components.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.